The present invention relates to an apparatus and method for detecting chemicals.
In industrial and biological processes, the importance of detecting and controlling carbon dioxide (CO.sub.2) concentrations continues to grow. The role of CO.sub.2 as a pollutant did not receive the attention it deserved until recently with the increasing discussion concerning the so-called "green house effect".
Some semiconductive, solid-state chemicals sensors have been discussed in the art, Technisches Messen, tm 56 (1989) Oldenbourg-Verlag, pp 260-63. These chemical sensors may be applied to industrial and biological applications. In addition, other fields where solid-state chemical sensors may be applied include:
monitoring ventilation and control of air conditioning and ventilation systems; PA1 monitoring air conditioning in green houses and other fields of agriculture; PA1 controlling biological and chemical processes in biotechnology; PA1 monitoring patients; and PA1 continuous and comprehensive emission control in industrial combustion processes.
Stricter regulations and growing public awareness with regard to the environment allow one to expect a rising demand for inexpensive CO.sub.2 sensors. In addition, concerns about chemical sensors utilized for the detection of carbon dioxide include alterations to the sensor material due to the influence of the gas to be detected. Thus, there is a need in the art for improved CO.sub.2 sensors.
Solid Electrolytic Sensors
A solid electrolytic carbon dioxide detecting sensor was developed by T. Maruyama and coworkers (Tokyo Institute of Technology, Japan). It is based on the Na.sup.+ ion conductor NASICON. Applied to it is a carbonate electrode, which reacts to changes in CO.sub.2 (1-3). When carbon dioxide is introduced, sodium ions form at the anode, which migrate through the carbonate to the electrolyte and reach the cathode. The resulting electromotive power (EMK) can be measured and the CO.sub.2 concentration in the sample is determined using the Nernst equation. Some of the major drawbacks of these sensors are the long response times, due to the fact that the rate of change in EMK depends on the diffusion rate of the ions, and that moisture influences the measured results. Another disadvantage is the need for a reference electrode since gas tight measurements are required. To date, it has not been possible to remedy the difficulties involved with a stable reference electrode.
Optical CO.sub.2 Sensors
Optical CO.sub.2 sensors have also been discussed. For example, a commonly used method for measuring CO.sub.2 is infrared absorption. This non-destructive method is based on the capability of CO.sub.2 molecules to absorb infrared radiation at specific wave lengths. The method, employing spectrometers, demonstrates stability and accuracy. Nevertheless, such methods and devices are very expensive and usually require complicated equipment. Due to the cost, this method of analysis can be ruled out for general monitoring in the field.
In other optical CO.sub.2 sensors, the change in the refractive index measured is a function of the CO.sub.2 concentration. At the Fraunhofer Institut fur Physikalische Me.beta.technik, in Freiburg, Germany an integrated optical sensor in which organically modified silicates are applied as a sensitive film onto an integrated optical interferometer (4), were developed under R. Edelhauser. The CO.sub.2 absorbed on this film caused its refractive index to change. However, the problems of selectivity, time constants, and reversibility can not, to date, be solved.
Another optical measuring method uses a suited indicator, which changes color under the influence of CO.sub.2. The intensity of this change in color depends on the gas concentration and is evaluated optically. In this case, an integrated solution is under development (C. H. Morgan, Microsensor Research Laboratory, University of Washington, USA (5) and not ready for general use.
Mass Sensitive Sensors
Gravimetric sensors absorb the gas to be measured on their surface reversibly. The settling of the substance to be examined on the surface of the sensor results in a change in mass and, consequently, in a change in the propagation velocity of the waves and a shift in the resonance frequency. At the Universitat Tubingen, Prof. W. Gopel's group coated a quartz microbalance sensor with a silicon based polymer in order to detect carbon dioxide (6). In Nieuwenhuizen et al., poly(ethyleneimine) was applied as a chemical interface for CO.sub.2 to a surface acoustic wave sensor (7). However, the sensor proved useless for measuring CO.sub.2 due to its great cross sensitivity to water and oxygen. At the same time, sensitivity to CO.sub.2 decreased with increasing use and an obvious baseline drift was observed. The characteristic frequency in this group of sensors is distinctly sensitive to temperature as well as to pressure. Thus, at the present state of development, such types of sensors are less suited for CO.sub.2 detection.
Ultrasonic Sensor
In this type of sensor, which was developed by V. M. Mecea (Institute of Isotopic and Molecular Technology, Cluj Napoca, Romania), a quartz crystal resonator generates ultrasonic waves in a cavity, which are reflected at the walls of the cavity. If the distance between the quartz surface and the parallel, reflecting walls is an integral multiplicity of the half wavelength, resonance occurs with the gas that is in the cavity and the entire oscillation energy of the resonator is absorbed by the gas. The resonance of CO.sub.2 can be obtained by setting the gap a specific length. If there is a small proportion of another gas in the gas flow, the resonance conditions change and this can be evaluated as a signal (8). The disadvantages of this sensor are the great drift in temperature and the lack of selectivity.
Capacitive Measuring Method
In these methods, the dielectric constant of the sensor material changes in that the molecules with a suitable dipole moment are absorbed at the surface of the sensor. However, CO.sub.2 does not have a dipole moment. Therefore, materials on which carbon dioxide can be chemisorbed are used for capacitive type CO.sub.2 sensors. As a consequence, the end product changes the dielectric constant. The degree of adsorption and the subsequent magnitude of the change in the dielectric constant depends on the CO.sub.2 concentration in the vicinity of the sensor. This change can be determined by means of the capacitor structure.
Under E. Obermeier (TU Berlin), a CO.sub.2 sensitive, organically modified silicate-based material was applied to thin film produced interdigitated capacitors. The dynamic properties of the sensors are extremely temperature dependent. At the same time, the sensor shows a great sensitivity to moisture (9, 10).
Research regarding a metal-oxide based capacitive measuring method for determining CO.sub.2 concentrations was published by a group connected to T. Isihara at the Oita University (Japan). The fundamental idea behind this sensor is based on the fact that the dielectric constants of the metal oxides principally differ from the constants of the metal carbonates (11-14). A powder composed of a mixture of different metal oxides is compressed to form tablets and centered. The electrodes are produced by applying silver to both sides of the tablet in order to obtain a capacitor structure. Wires inserted into the silver droplets are the contact. A rise in the ambient CO.sub.2 concentration leads to an increase in sensor capacity. At the same time, water plays a significant role due to its large dielectric constant. This type of CO.sub.2 sensor design proves to be very disadvantageous. In sensors, the use of tablets is impractical because they break easily. Resort to the non-reproducible dripping of silver for producing the electrodes, because of the difficulties in contacting tablets made of compressed powder, complicates matters. Even sputtering methods would not lead to solderable or bondable silver due to the thinness of the layer required.
Many other disadvantages of this method exist. The production of the surface and configuration of the electrodes is imprecise and irreproducible. Consequently, sensor capacity as a function of the electrode surface is also irreproducible. It is as if the chemically active layer would be largely shielded from the surroundings by the electrodes. The ratio of volume to surface of the sensor body is very big and the diffusion path is long. Due to the great distance of 0.6mm between the electrodes as a result of the thickness of the tablet, the sensor has a high impedance. The high setting temperature, of approximately 475.degree., is also required for CO.sub.2 detection and has to be executed using an external heating device. Also, detecting the changes in dielectric properties requires a complicated evaluation circuit. Clearly, sensors in tablet form are not suited for mass production.
Polymer Conductive Sensors
The principle of these sensors is based on a change in conductivity due to gas adsorption and the subsequent surface reaction. A certain selectivity is obtained by the choice of the catalyst (15). Preliminary experiments with polymer conductivity sensors were conducted on the basis of PPA (polyphenylacetylene) (16). To date, it has not been possible to put this sensor principle into practice.
The present invention provides a solid-state chemical sensor in which reference electrodes are obviated. The sensor is substantially cheaper than others and permits the determination of close meshed concentrations of pollutants in industry, at the workplace, or in traffic with sufficient accuracy, but at relatively low cost. Monitoring the CO.sub.2 concentration in the environment makes it possible to detect environmental hazards and to initiate pinpoint countermeasures.
The solid-state sensor of the invention comprises, for example, a semiconductive solid-state chemical sensor having a ceramic carrier with a heating element having branched circuit connections disposed on one side, and interdigitated electrodes, on the other side. A gas sensitive material is disposed, using thick film technology, on the electrode side. For a CO.sub.2 sensor, the gas sensitive material may comprise, for example, CuO and Ba-TiO.sub.3 and additional metal oxides as catalysts and/or adhesion compounds in a sintered form. A film thickness of approximately 20-200 .mu.m is used and conductivity is measured by means of the resistance dependent upon the change in the CO.sub.2 concentration.
In other embodiments, the gas sensitive material has a grain size of less than 5 .mu.m, and preferably in the nano structure range of 1nm to 400 nm. In addition, catalysts may optionally be added to the gas sensitive material at 0.1 to 10%.
Also, a Cu.sub.x Ce.sub.y O.sub.z may be employed as metal oxide in the gas sensitive material, with x, y, and z being in the range of approximately 1-5, with x, y, and z not necessarily being integers.
In other embodiments, the interdigitated electrodes may be made of gold or another precious metal. And the heating electrodes may be made of platinum or palladium.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.